Python in 2.80: How to get the base color texture from a Material
For a simple setting, you can use this:

import bpy
from mathutils import Color


# try/except if the setting is not as expected
try:
    # Get active object
    obj = bpy.context.active_object

    # Get its first material slot
    material = obj.material_slots[0].material
    # Get the nodes in the node tree
    nodes = material.node_tree.nodes
    # Get a principled node
    principled = next(n for n in nodes if n.type == 'BSDF_PRINCIPLED')
    # Get the slot for 'base color'
    base_color = principled.inputs['Base Color'] #Or principled.inputs[0]
    # Get its default value (not the value from a possible link)
    value = base_color.default_value
    # Translate as color
    color = Color( (value[0], value[1], value[2]) )
    print( color )

    # Get the link
    link = base_color.links[0]
    link_node = link.from_node
    print( link_node.image.name )
except:
    print( 'not found' )